Abstract
What is the central question of this study? What is the absolute level of pre-exercise glycogen concentration required to augment the exercise-induced signalling response regulating mitochondrial biogenesis? What is the main finding and its importance? Commencing high-intensity endurance exercise with reduced pre-exercise muscle glycogen concentrations confers no additional benefit to the early signalling responses that regulate mitochondrial biogenesis. We examined the effects of graded muscle glycogen on the subcellular location and protein content of AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and mRNA expression of genes associated with the regulation of mitochondrial biogenesis and substrate utilisation in human skeletal muscle. In a repeated measures design, eight trained male cyclists completed acute high-intensity interval (HIT) cycling (8×5min at 80% peak power output) with graded concentrations of pre-exercise muscle glycogen. Following initial glycogen-depleting exercise, subjects ingested 2gkg-1 (L-CHO), 6gkg-1 (M-CHO) or 14gkg-1 (H-CHO)of carbohydrateduring a 36hperiod, such that exercise was commenced with graded (P<0.05) muscle glycogen concentrations (mmol(kgdw)-1 : H-CHO, 531±83; M-CHO, 332±88; L-CHO, 208±79). Exercise depleted muscle glycogen to <300mmol(kgdw)-1 in all trials (mmol(kgdw)-1 : H-CHO, 270±88; M-CHO, 173±74; L-CHO, 100±42) and induced comparable increases in nuclear AMPK protein content (∼2-fold)and PGC-1α (∼5-fold), p53 (∼1.5-fold) and carnitine palmitoyltransferase 1 (∼2-fold) mRNA between trials (all P<0.05). The magnitude of increase in PGC-1α mRNA was also positively correlated with post-exercise glycogen concentration (P<0.05). In contrast, neither exercise nor carbohydrate availability affected the subcellular location of PGC-1α protein or PPAR, SCO2, SIRT1, DRP1, MFN2 or CD36 mRNA. Using a sleep-low, train-low model with a high-intensity endurance exercise stimulus, we conclude that pre-exercise muscle glycogen does not modulate skeletal muscle cell signalling.
Highlights
We demonstrate that commencing an acute bout of work-matched and non-exhaustive high-intensity interval (HIT) cycling with graded pre-exercise muscle glycogen (within a range of 600–200 mmol−1) does not modulate such early signalling responses
In the context of manipulating CHO availability around training, our data suggest that the metabolic stress of HIT exercise may override any potential effect of pre-exercise muscle glycogen and induce negligible modulatory effects on skeletal muscle that is already subjected to the local metabolic challenge of high-intensity exercise
We utilised an experimental protocol consisting of a purposeful amalgamation of previous train-low protocols whereby participants perform a glycogen depletion protocol on the evening of Day 1, consume a modified CHO intake throughout Day 2 and perform fasted exercise on the morning of Day 3
Summary
The concept of deliberately commencing endurance exercise with reduced muscle glycogen (i.e. the train-low paradigm, Burke et al, 2018) is recognised as a potent nutritional strategy that is able to modulate acute skeletal muscle cell signalling (Bartlett et al, 2013; Wojtaszewski et al, 2003; Yeo et al, 2010) and transcriptional responses (Bartlett et al, 2013; Pilegaard et al, 2002; Psilander, Frank, Flockhart, & Sahlin, 2013). Skeletal muscle glycogen appears to exert its regulatory effects primarily through the AMP-activated protein kinase (AMPK)– peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α) signalling axis, whereby exercise-induced AMPKα2 activity (Wojtaszewski et al, 2003), phosphorylation (Yeo et al, 2010) and nuclear abundance (Steinberg et al, 2006) are all augmented under conditions of reduced pre-exercise muscle glycogen These effects may be partly mediated through the glycogen binding domain present on the β subunit of AMPK (McBride & Hardie, 2009; McBride, Ghilagaber, Nikolaev, & Hardie, 2009). Endurance exercise appears to increase the nuclear abundance of PGC-1α (Little, Safdar, Cermak, Tarnopolsky, & Gibala, 2010; Little, Safdar, Bishop, Tarnopolsky, & Gibala, 2011), and may constitute the initial phase of exercise-induced adaptive responses
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